For slab or channel waveguides, it is necessary that the material through which the light is propagated have an index of refraction larger than that of surrounding media However, in order that the light is propagated along and is confined to the slab or channel material there are more stringent requirements. Generally, modes of propagation are classified into two kinds according to the orientation of the field vectors those with transverse electric fields (TE modes) and those with transverse magnetic fields (TM modes). The solutions to Maxwell's equations for these modes for a slab waveguide are well-known (see e.g. "Optical Waves in Crystals", Yariv, et al., John Wiley & Sons, New York, Chapter 11 (1984); or "Integrated Optics: Theory and Technology", Hunsperger Springer-Verlag, Berlin 16-37 (1982). The number of such confined modes depends on the frequency of the light wave, the depth of the slab, and the indices of refraction of the three media involved, i.e., that of the substrate or plate material n.sub.s, that of the slab waveguide material, n.sub.s +.DELTA.n, and that of the material above the top surface of the slab waveguide, n.sub.a. For a given frequency, the number of confined modes increases with increasing slab depth or with increasing index of refraction of the slab (i.e., with increasing .DELTA.n). For an extremely thin slab or an extremely small .DELTA.n, no mode is confined. As the depth of the slab and/or .DELTA.n is increased, one mode becomes confined, then another, etc.
The solutions of Maxwell's equations for confined modes for a channel waveguide are more complicated than for a slab waveguide. For this reason, only approximate solutions have been obtained (see "Integrated Optics: Theory and Technology", Hunsperger, Springer-Verlag, Berlin, 38-43 (1982)). For a given value of .DELTA.n, there are certain minimum values for the depth and width of the channel in order for the channel to be able to confine a mode. These depth and width values are not independent, i.e., to confine a given mode, wider channels can be less deep while narrower channels require a greater depth. Typically, the depths and widths of approximately square channel waveguides are several times the depth of a slab waveguide. For both slab and channel waveguides, the index of refraction must be large enough so that at least one mode is confined.
Another more recently developed form of a waveguide involves periodic modulation of the refractive index of the waveguide surface, such as that described in U.S. Pat. No. 5,028,107.
There has been considerable interest in providing crystals suitable for use in the products of optical devices. Potassium Titanyl Phosphate (i.e., KTP) and certain analogs thereof are of particular note. For example, U.S. Pat. No. 3,949,323 discloses crystals of compounds having the formula MTiO(XO.sub.4), wherein M is at least one of K, Rb, Tl, or NH.sub.4 and X is at least one of P or As and wherein X is P when M is NH.sub.4 and nonlinear optical devices and electro-optic modulators which use such crystals. U.S. Pat. No. 4,231,838 discloses a process for the manufacture of single crystals of MTiOXO.sub.4, wherein M is K, Rb or Tl and X is P or As, of optical quality and of sufficient size for use in nonlinear optical devices, said process comprising the steps of heating certain starting ingredients (chosen to be within the region of a ternary phase diagram in which the desired crystal MTiOXO.sub.4 product is the only stable solid phase) to produce MTIOXO.sub.4, and then controllably cooling to crystallize the desired product. Crystals which have mixtures of elements for M and/or X can be grown by the process. U.S. Pat. No. 4,305,778 discloses a hydrothermal process for growing single crystals of MTiOXO.sub.4, wherein M is K or Rb and X is P or As, said process involving using as a mineralizing solution an aqueous solution of a glass defined by specified portions of the ternary diagrams for the selected M.sub.2 O/X.sub.2 O.sub.5 /(TiO.sub.2).sub.2 system.
Methods of modifying KTP and certain analogs thereof to produce optical waveguides have also been studied. For example, U.S. Pat. No. 4,740,265 and U.S. Pat. No. 4,766,954 disclose a process for producing an optical waveguide comprising contacting at least one optically smooth surface of a single crystal of a K.sub.1-x Rb.sub.x TiOMO.sub.4 (wherein x is from 0 to 1 and M is P or As) with a specified molten salt of at least one of Rb, Cs, and Tl at a temperature of from about 200.degree. C. to about 600.degree. C. for a time sufficient to increase the surface index of refraction at least about 0.00025 with respect to the index of refraction of the starting crystal, and cooling the resulting crystal. U.S. Pat. No. 5,028,107 describes periodically modifying crystals to produce a waveguide useful for wavelength conversion.
M. M. Eddy et al., Inorg. Chem. 27, 1856 (1988) describes preparation and some properties of compounds of the formula NH.sub.4 TiOPO.sub.4 (NTP), NH.sub.4 H(TiOPO.sub.4).sub.2 (NHTP), H.sub.3 ONH.sub.4 (TiOPO.sub.4).sub.2 (NOTP), and H.sub.2 (TiOPO.sub.4).sub.2 in powdered form.
R. H. Jarman, Solid State Ionics 32/33, 45 (1989) describes ion exchange reactions using KTiOPO.sub.4 and molten salts containing sodium, lithium and hydrogen ions. Partial ion-exchange of KTiOPO.sub.4 in excess molten NaNO.sub.3 was achieved. In contrast, Li salts under the same conditions did not affect ion-exchange, but instead caused decomposition of the KTP latice; and proton exchange in inorganic or organic media was not achieved.
Other crystal systems have also been studied in connection with optical waveguides. For example, Wong, SPIE Vol 993 Integrated Optical Circuit Engineering VI, pages 13-25 (1988) reviews a proton exchange processes for producing proton exchanged LiNbO.sub.3 and LiTaO.sub.3 waveguides.